Support start trimming post-decoding. Use it with FFmpegDemuxer.

media/base/audio_discard_helper.h

Index: media/base/audio_discard_helper.h
diff --git a/media/base/audio_discard_helper.h b/media/base/audio_discard_helper.h
index 388cbd421a380f8ac5646ac3c87dc060bb04774f..deeb45f37ea8eef4600f0142bfbe7ec1eef0ba3f 100644
--- a/media/base/audio_discard_helper.h
+++ b/media/base/audio_discard_helper.h
@@ -9,17 +9,31 @@
 #include "base/time/time.h"
 #include "media/base/audio_timestamp_helper.h"
 #include "media/base/buffers.h"
+#include "media/base/decoder_buffer.h"
 #include "media/base/media_export.h"
 
 namespace media {
 
 class AudioBuffer;
-class DecoderBuffer;
 
 // Helper class for managing timestamps and discard events around decoding.
 class MEDIA_EXPORT AudioDiscardHelper {
  public:
-  explicit AudioDiscardHelper(int sample_rate);
+  // |sample_rate| is the sample rate of decoded data which will be handed into
+  // the ProcessBuffers() call.
+  //
+  // |decoder_delay| is the number of frames a decoder will output before data
+  // corresponding to the first encoded buffer is output.  Callers only need to
+  // specify this if the decoder inserts frames which have no corresponding
+  // encoded buffer.
+  //
+  // For example, most MP3 decoders will output 529 junk frames before the data
+  // corresponding to the first encoded buffer is output.  These frames are not
+  // represented in the encoded data stream and instead are an artifact of how
+  // most MP3 decoders work.  See http://lame.sourceforge.net/tech-FAQ.txt
+  //
+  // NOTE: End discard is only supported when there is no |decoder_delay|.
+  AudioDiscardHelper(int sample_rate, size_t decoder_delay);
   ~AudioDiscardHelper();
 
   // Converts a TimeDelta to a frame count based on the constructed sample rate.
@@ -50,11 +64,15 @@ class MEDIA_EXPORT AudioDiscardHelper {
 
  private:
   const int sample_rate_;
+  const size_t decoder_delay_;
   AudioTimestampHelper timestamp_helper_;
 
   size_t discard_frames_;
   base::TimeDelta last_input_timestamp_;
 
+  bool delayed_discard_;
+  DecoderBuffer::DiscardPadding delayed_discard_padding_;
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(AudioDiscardHelper);
 };